Meniscus Prosthetic Device

ABSTRACT

A prosthetic device that may be utilized as an artificial meniscus is disclosed. The prosthetic device can restore shock absorption, stability, and function to the knee after the damaged natural meniscus is removed and replaced with the prosthetic device. In some embodiments, the meniscus includes an integral fixation anchor and additional features that minimize the requirement for modification of the implant for proper fit during surgery.

PRIORITY

This is a continuation application claiming priority to U.S. patentapplication Ser. No. 11/868,254 filed Oct. 5, 2007, which claimspriority to U.S. Provisional Application No. 60/828,770 filed Oct. 9,2006, each of which is hereby incorporated by reference in its entirety.

FIELD/BACKGROUND

The present disclosure generally relates to medical prosthetic devicesthat replace the functionality of the natural meniscus. Each knee hastwo menisci, a lateral meniscus and a medial meniscus. Each meniscus isa crescent-shaped fibrocartilaginous tissue attached to the tibia at ananterior and a posterior horn. Damage to the meniscus can causedebilitating pain and arthritis. In some instances the prostheticdevices of the present disclosure are configured to be surgicallyimplanted into a knee joint to replace the natural meniscus.

SUMMARY

In one embodiment, a meniscus prosthesis is disclosed.

In another embodiment, a meniscus prosthesis is disclosed that achievesgood tribology by maintaining precise contact surfaces between themeniscus, the femoral condyle cartilage, and the tibial plateaucartilage. This is accomplished through the use of a hollow-likestructure that is capable of the deformation necessary to accommodatethe cartilage surfaces. The prosthesis can advantageously utilize modernmaterial technology in a configuration that, in addition to itsoutstanding physical characteristics, significantly reduces the need forcustomization and fitting of the prosthesis during the implantationprocedure. The meniscus prosthesis can be manufactured in a sufficientrange of sizes to fit all applications. In some embodiments, the meansfor mechanical fixation of the implant to the tibial platform may beintegrated into the meniscus body eliminating the requirement for themeniscus to be connected to an intermediary, separate fixation meanssuch as a bone bridge. The fixation anchor configuration can utilize akeyhole cross section geometry that provides secure position controlwhile minimizing lateral stresses that may result from the bone screwsecuring techniques of prior approaches.

In another embodiment, a meniscus prosthetic device comprises asemi-ellipsoidal solid body structure having top and bottom surfaces.The top surface is concavely shaped to mate with a surgically preparedfemoral condyle and the bottom surface is shaped to mate with asurgically prepared tibial plateau. The top and bottom surfaces define ashelf comprising a membrane section that extends between the interior ofthe semi-elliptical walls of the body structure. The thickness of themembrane section may be less than approximately 2 millimeters and theheight of the semi-elliptical walls may be less than approximately 15millimeters. The cross section of the walls of the body structurefunctionally duplicates the nominal cross section of the naturalmeniscus. A fixation anchor may be integrated into and extend from thebottom surface of the prosthetic device.

In some embodiments, the fixation anchor comprises a keel having akeyhole shaped cross section. The anchor may extend substantiallyparallel to the bottom surface of the prosthetic device. The anchor maybe in close proximity to the edge of the shelf in some embodiments. Thekeel may either continuously or discontinuously extend across the widthof the bottom surface. In addition, the fixation anchor may comprise oneor more tabs projecting from and perpendicular to the bottom surface. Ina further embodiment, the meniscus may comprise a non-biologicallyderived material such as a pliable polyurethane based polymer. Inanother implementation, the meniscus may further include a deformationcontrol element integrated into the meniscus that may, for example, be afilament wound into a machined undercut in the lower surface. In anotherembodiment, the bottom surface of the meniscus may be coated with abioactive coating applied for the purpose of encouraging the in-growthof natural tissue into the meniscus. Such in-growth may improve fixationof the replacement meniscus to the tibial plateau.

BRIEF DESCRIPTION OF DRAWINGS

Other features and advantages of the present disclosure will becomeapparent in the following detailed description of embodiments of thedisclosure with reference to the accompanying of drawings, of which:

FIG. 1 is a diagrammatic perspective view of an embodiment of aprosthetic device according to one embodiment of the present disclosure.

FIG. 2 is an alternative diagrammatic perspective view of the prostheticdevice of FIG. 1.

FIG. 3 is an alternative diagrammatic perspective view of the prostheticdevice of FIGS. 1 and 2.

FIG. 4 is an alternative diagrammatic perspective view of the prostheticdevice of FIGS. 1, 2, and 3.

FIG. 5 is a diagrammatic cross-sectional view of the prosthetic deviceof FIGS. 1, 2, 3, and 4.

FIG. 6 is a diagrammatic side view of an arrangement showing theprosthetic device of FIGS. 1, 2, 3, and 4 inserted into a surgicallyprepared knee joint.

FIG. 7 is a diagrammatic front view of the arrangement of FIG. 6.

FIG. 8 is a diagrammatic cross-sectional view of a prosthetic devicesimilar to FIG. 5, but showing an alternative embodiment.

FIG. 9 is a diagrammatic cross-sectional view of a prosthetic devicesimilar to FIGS. 5 and 8, but showing an alternative embodiment.

FIG. 10 is a diagrammatic cross-sectional view of a prosthetic devicesimilar to FIGS. 5, 8, and 9, but showing an alternative embodiment.

FIG. 11 is a diagrammatic cross-sectional view of a prosthetic devicesimilar to FIGS. 5, 8, 9, and 10 but showing an alternative embodiment.

FIG. 12 is a diagrammatic cross-sectional view of a prosthetic devicesimilar to FIGS. 5, 8, 9, 10, and 11, but showing an alternativeembodiment.

FIG. 13 is a diagrammatic cross-sectional view of a prosthetic devicesimilar to FIGS. 5, 8, 9, 10, 11, and 12 but showing an alternativeembodiment.

FIG. 14 is a diagrammatic cross-sectional view of the prosthetic deviceof FIG. 13 with an insert according to one embodiment of the presentdisclosure.

FIG. 15 is a diagrammatic cross-sectional view of a prosthetic deviceFIG. 13 with an insert similar to FIG. 14, but showing an alternativeembodiment of the insert.

FIG. 16 is a diagrammatic perspective view of a prosthetic deviceaccording to another embodiment of the present disclosure.

FIG. 17 is a diagrammatic cross-sectional view of the prosthetic deviceof FIG. 16.

FIG. 18 is a diagrammatic perspective view of a prosthetic deviceaccording to another embodiment of the present disclosure.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the disclosure is intended. Any alterations and furthermodifications in the described devices, instruments, methods, and anyfurther application of the principles of the disclosure as describedherein are contemplated as would normally occur to one skilled in theart to which the disclosure relates. In particular, it is fullycontemplated that the features, components, and/or steps described withrespect to one embodiment may be combined with the features, components,and/or steps described with respect to other embodiments of the presentdisclosure.

Referring now to FIGS. 1, 2, 3, 4, and 5 shown therein is a prostheticdevice 10 according to one aspect of the present disclosure. Inparticular, FIGS. 1, 2, 3, and 4 are various perspective views of thedevice 10. FIG. 5 is a cross-sectional view of the device 10. Generally,the prosthetic device 10 is for the replacement of a meniscus that hasbeen damaged, ruptured, disintegrated, diseased, or is otherwise in needof replacement. For illustrative purposes, the prosthetic device 10 willbe described for use with a left knee, lateral meniscus replacement.However, corresponding embodiments may be utilized for replacement ofany of the other menisci, such as the left knee medial meniscus, rightknee lateral meniscus, and/or right knee medial meniscus. In thatregard, the position, size, shape, and/or other properties of thefixation anchor may be configured for each particular application.Similarly, the size, shape, thickness, material properties, and/or otherproperties of the prosthetic device may be configured for eachparticular application.

The prosthetic meniscus 10 comprises an outer body portion 12, a centralbody portion 14, a fixation member 16, and a fixation device 18.Generally, the outer body portion 12 has an increased thickness andheight relative to the central body portion 14. In some instances theouter body portion 12 has a thickness between 5 mm and 15 mm. In someinstances, the central body portion 14 has a thickness between 0.1 mmand 5 mm. In one particular embodiment, the outer body portion 12 has athickness of approximately 10 mm and the central body portion 14 has athickness of approximately 2 mm. Further, in some instances the outerbody portion 12 has an increased stiffness relative to the central bodyportion 14. As discussed in greater detail below, this increasedstiffness may be a result of different material properties, geometries,support features, and/or other mechanisms for varying the stiffnessbetween the central body portion 14 and the outer body portion 12.

Generally, the central body portion 14 defines an upper articulationsurface 20 and a lower fixation surface 22. The fixation member 16extends from the lower fixation surface 22. The upper articulationsurface 20 is bounded by the outer body portion 12 on several sides. Inthat regard, the outer body portion 12 comprises a rim or wall having anincreased height relative to the central body portion 14 such that thecentral body portion is recessed with respect to the outer body portion.In the current embodiment, the outer body portion 12 defines asubstantially convex upper surface 24 that tapers down in to the upperarticulation surface 20 on one side and to an outer surface 26 of theprosthetic device 10 on the other side. Accordingly, the upper surface20 of the central body portion 14 and the taper of the upper surface 24of the outer body portion 12 define a concave recess configured forreceiving a portion of the femur such as the femoral condyle. The outerbody portion 12 has a semi-ellipsoidal shape in some embodiments. In oneparticular embodiment, the outer body portion 12 is shaped tosubstantially match the shape of a natural meniscus.

While the majority of the central body portion 14 is bounded by theouter body portion 12, one side of the body portion 12 defines an edgeor boundary 28. In the current embodiment, the boundary 28 is asubstantially planar surface having a thickness approximately equal tothe thickness of the central body portion 14. In other embodiments, theboundary 28 may have an increased thickness relative to the central bodyportion 14 (e.g., see FIGS. 16 and 17). In some embodiments, theboundary 28 has a thickness greater than the central body portion 14,but less than the outer body portion 12. In one particular embodiment,the boundary 28 may have a thickness such that it extends above theupper surface 20 approximately one-half of the distance of the uppersurface 24 of the outer body portion 12.

As noted above, the fixation member 16 extends down from the lowersurface 22 of the prosthetic device 10. In the current embodiment, thefixation member 16 extends from the lower surface 22 adjacent to andsubstantially parallel to the boundary 28. In other embodiments, thefixation member 16 may extend from other portions of the prostheticdevice 10 and/or in other directions, including directions substantiallyperpendicular to the boundary 28 and/or oblique to the boundary 28.Alternative positioning and orientations of the fixation member 16 areused to accommodate alternative surgical approaches, patient specificanatomical attributes, meniscus specific orientations, physicianpreference, and/or other factors.

In the current embodiment, the fixation member 16 comprises a keelstructure having a first portion 30 extending directly from the lowersurface 22 and a second portion 32 extending from the first portion 30.The second portion 32 has an increased profile or thickness relative tothe first portion 30. In the current embodiment, the fixation member 16has a keyhole cross section that can engage a complementary keyholeshaped groove that has been surgically incised a portion of the tibia,such as the tibia plateau, according to a keyhole surgical approach. Inthat regard, the fixation member 16 is configured to engage an openingextending substantially in a direction from the anterior to theposterior of the tibia. In other embodiments, the fixation member 16 mayhave other structural geometries to encourage engagement with the tibia.In one particular embodiment, the fixation member 16 comprises adovetail configured to engage a dovetailed groove prepared in the tibia.

The fixation member 16 is manufactured as an integral part of theprosthetic device in some embodiments. In that regard, the fixationmember 16 may be molded simultaneously with the other portions of theprosthetic device 10 and/or permanently attached to the other portionsof the prosthetic device. Implementing the fixation member 16 as anintegral component of the prosthetic device 10 provides several distinctadvantages. First, the configuration and dimensions of the fixationmember 16 are precisely controlled prior to the surgical procedure andmay be maintained even for different size prosthetic devices 10.Accordingly, a single size of standardized surgical tools, includingbores, rasps and guides, etc., may be employed for the implantation ofany size prosthetic device 10. This results in a more precise engagementand mating between the prosthetic device 10 and the tibia, whichimproves the fixation properties and overall performance of theprosthetic device. Second, by implementing the fixation member 16 as anintegrated section of the overall prosthetic device 10, the possibilityof separation of the fixation member 16 from the other portions of theprosthetic device 10 is virtually eliminated. Third, the keyhole and/ordovetail configurations of the fixation member 16 do not require the useof bone screws or other means to tighten or secure the fixation member16 to the tibia. Accordingly, the integral fixation member 16 minimizesor eliminates the need to customize the fixation member, therebyreducing the time required to perform the implantation procedure.

In some embodiments, a fixation device 18 is utilized in combinationwith the fixation member 16 to secure the prosthetic device 10 to thetibia. It should be noted, however, that in other embodiments, thefixation member 16 is the sole fixation means utilized to secure theprosthetic device 10 to the tibia. The fixation device 18 extends fromthe bottom surface 22. As shown, in some embodiments the fixation device18 extends substantially perpendicularly to the bottom surface 22. Thefixation device 18 is utilized to provide further fixation of theprosthetic device 10 to the tibia. In that regard, in the currentembodiment the fixation device 18 includes an opening 34 for receiving afixation member. The fixation member may be a bone screw, staple, orother device configured to secure the prosthetic device 2 to the tibiathrough the opening 34. In one particular embodiment, the fixationdevice 18 may be fastened to the tibia by means of a bone screwextending through the opening and securely engaging the tibia.

When the prosthetic device 10 is implanted and secured to the tibia, thecentral body portion 14 bounded by the outer body portion 12 serves toisolate the femoral condyle from the tibial plateau when implanted intoa patient. In that regard, the outer body portion 12 serves to limit themovement of the femoral condyle relative to the prosthetic device. Inparticular, in the current embodiment the outer body portion 12 preventsthe portion of the femur movingly engaged with the prosthetic device 10from moving laterally beyond outer body portion. In other embodiments,the outer body portion 12 limits movement of the femur relative to theprosthetic device 10 in the medial direction. Further, the prostheticdevice 10 provides shock absorption and a desirable tribology betweenthe femur and tibia thereby attributing to the overall therapeutic valueof the prosthesic device.

The prosthetic device 10 may be manufactured in various sizes, so thatany given application can be satisfied by a “stock” unit. Accordingly, asurgeon could, during an implantation procedure, select a correctlysized device from the selection of stock units. Alternatively, inanother embodiment, a replacement meniscus could be custom manufacturedfor a particular patient utilizing characteristics determined by medicalimaging techniques, such as MRI, coupled with computer aidedmanufacturing (CAM) techniques.

In some embodiments, the bottom surface 22 of the prosthetic device 10is coated with a bioactive coating to encourage the in-growth of naturaltissue to further improve fixation of the prosthetic device to thetibial plateau. In some embodiments, the coating is formed by gritblasting or spraying the bottom surface 22. The bioactive coating may beany suitable material for encouraging tissue growth and, in someembodiment, may be specifically adapted for promoting bone growthbetween the tibia and the prosthetic device 10.

Referring generally to FIGS. 8-15, in some embodiments the outer bodyportion 12 of the prosthetic device 10 includes a deformation controlelement to limit the deformation of the outer body portion. As will bedescribed in greater detail with respect to the specific embodimentsshown in FIGS. 8-15, the deformation control element may be a materialproperty, a structural property, an additional component, and/orcombinations thereof. It should be noted that the various deformationcontrol elements described herein may be combined to further limit ordefine the amount of deformation of the outer body portion 12.

Referring more particularly to FIG. 8, shown therein is across-sectional view of the prosthetic device 10 wherein the outer bodyportion 12 is comprised of a reinforced material relative to the centralbody portion 14. For example, in one embodiment the outer body portion12 includes carbon fibers providing additional strength and limiting theflexibility of the outer body portion. In some embodiments the carbonfibers are injected prior to the curing of the outer body portion 12. Inother embodiments, the outer body portion 12 is formed or molded aroundthe carbon fibers. In other embodiments, other additives are utilized toreinforce the material of the outer body portion 12. The particularadditives that are used depend upon the material(s) used for forming theouter body portion 12. As shown in FIG. 8, in the current embodiment theentire outer body portion 12 is formed from a substantially uniformmaterials and/or the additives are equally distributed throughout theouter body portion. However, in other embodiments the deformationcontrol element may comprise only a portion of the outer body portion12. In that regard, the deformation control element may extend alongonly a portion of the outer body portion 12, the deformation controlelement may be positioned within a particular portion of the outer bodyportion, and/or combinations thereof.

For example, referring more particularly to FIG. 9, shown therein is across-sectional view of the prosthetic device 10 wherein the outer bodyportion 12 includes a wire, cable, or filament 38 extendingtherethrough. The filament increases the stiffness of the outer bodyportion 12 to limit the flexibility and/or deformity. In someembodiments, the filament 38 comprises a carbon fiber. In otherembodiments, the filament 38 comprises a metal, polymer, or othermaterial having an increased hardness and/or stiffness relative to thematerial comprising the central body portion 14. In some embodiments,the outer body portion 12 is formed around the filament 38. In otherembodiments, the filament 38 is inserted into the outer body portion 12prior to curing of the prosthetic device 12. In some embodiments, thefilament 38 is inserted into an opening in the body portion 12 and thenadditional material is inserted into the opening to close the openingand secure the filament therein. In the current embodiment, the filament38 is shown having a substantially circular or cylindricalcross-section. However, in other embodiments the filament 38 may haveother geometrical cross-sections and/or varying cross-sections along itslength. The cross-section(s) of the filament 38 are configured toprovide the desired stiffness and deformation properties to the outerbody portion 12.

Further in the current embodiment, the outer body portion 12 has a totalthickness or height H₁, the filament 38 is positioned a distance H₂ fromthe peak of the upper surface 24, and the filament has a height H₃. Insome embodiments, the total thickness H₁ of the outer body portion 12 isbetween 5 mm and 15 mm. In some embodiments, the thickness H₃ of thefilament 38 is between ¼ and ½ of the total thickness H₁ of the outerbody portion 12. The distance H₂ from the peak of the upper surface 24to the filament 38 varies from 0 (i.e., the filament 38 is positioned atthe top of the outer body portion 12) to ¾ of the total thickness H₁(i.e., the filament is positioned at the bottom of the outer bodyportion). In that regard, in some embodiments the filament 38 mayengaged a recess in the upper surface 24 or the lower surface of theouter body portion 12 configured to receive the filament 38. In oneparticular embodiment, the total thickness H₁ is approximately 10 mm,the distance H₂ is approximately 3.3 mm, and the thickness H₃ isapproximately 3.3 mm.

Similarly, in the current embodiment the outer body portion 12 has atotal thickness or width W₁, the filament 38 is positioned a distance W₂from the outer surface 26, and the filament has a width W₃. In someembodiments, the total width W₁ of the outer body portion 12 is between5 mm and 15 mm. In some embodiments, the width H₃ of the filament 38 isbetween ¼ and ½ of the total width W₁ of the outer body portion 12. Thedistance W₂ from the peak of the upper surface 24 to the filament 38varies from 0 (i.e., the filament 38 is positioned at the very inside ofthe outer body portion 12) to ¾ of the total thickness W₁ (i.e., thefilament is positioned at the outside of the outer body portion). Inthat regard, in some embodiments the filament 38 may engaged a recess inthe outer surface 26 or the inner surface of the outer body portion 12configured to receive the filament 38. In one particular embodiment, thetotal width W₁ is approximately 10 mm, the distance W₂ is approximately3.3 mm, and the width W₃ is approximately 3.3 mm. In some embodiments,the outer body portion 12 may include multiple filaments 38 positionedtherein. In that regard, the multiple filaments 38 may be spaced equallyabout the outer body portion 12 and/or grouped into specific areas ofthe outer body portion.

Further, in the illustrated embodiment the fixation member 16 includes afilament 39 extending therethrough. The filament 39 increases thestiffness of the fixation member 16. In the current embodiment, thefilament 39 extends substantially along the length of the fixationmember 16 through portion 32. In other embodiments, the fixation member16 may include other features that increase the stiffness of thefixation member. In particular, the fixation member 16 may includefeatures similar to those described with respect to the deformationcontrol elements of the outer body portion 12.

Referring now to FIG. 10, shown therein is a cross-sectional view of theprosthetic device 10 wherein the outer body portion 12 includes area 40of increased stiffness and/or hardness. In the current embodiment thearea 40 extends substantially across the entire width of the outer bodyportion 12. The area 40 may comprise a different material from the restof the outer body portion, the same material as the rest of the outerbody portion with additives, and/or an insert piece configured to besecured within the outer body portion. As mentioned with respect to theother deformation control elements, the position, size, and shape of thearea 40 is configured to achieve the desired deformation properties forthe outer body portion 12. For example, referring to FIG. 11, showntherein is a cross-sectional view of the prosthetic device 10 whereinthe outer body portion 12 includes an area 42 of increased stiffnessand/or hardness extending substantially along the entire thickness orheight of the outer body portion.

Referring now to FIG. 12, shown therein is a cross-sectional view of theprosthetic device 10 wherein the outer body portion 12 includes a recess44 for receiving a component 46 for defining the deformation propertiesof the outer body portion. For example, in some instances the component46 may be a wire, cable, or filament similar to the filament 38described above. In other instances, the component 46 may be a materialthat is injected or otherwise introduced into the recess 44 in the outersurface 26. Generally, the size of the recess 44 and the properties ofthe component 46 are tailored to achieve the desired deformationproperties of the outer body portion 12. In some embodiments, the recess44 comprises between ⅛ and ⅔ of the height of the outer body portion 12and between ⅛ and ⅔ of the width of the outer body portion. In manyembodiments, the component 46 substantially fills the entire recess 44.However, in some embodiments the component 46 is sized such that itfills only a portion of the recess 44. In such embodiments, theremaining portion of the recess 44 may remain vacant or be filled withanother material. In some embodiments, the component 46 is secured inthe recess 44 by the introduction of additional material into the openspace remaining in the recess.

Referring now to FIGS. 13 and 14, shown therein is a cross-sectionalview of the prosthetic device 10 wherein the outer body portion 12includes an undercut 48 extending upward from the bottom surface of theouter body portion. In some instances the undercut 48 is configured toreceive a component 50 for increasing the stiffness and limiting thedeformation of the outer body portion 12. For example, in some instancesthe component 50 may be a wire, cable, or filament similar to thefilament 38 described above. In other instances, the component 50 may bea material that is injected or otherwise introduced into the recess 48in the outer surface 26. Generally, the size of the recess 48 and theproperties of the component 50 are selected to achieve the desireddeformation properties of the outer body portion 12. In someembodiments, the recess 48 comprises between ⅛ and ⅞ of the height ofthe outer body portion 12 and between ⅛ and ⅞ of the width of the outerbody portion. In many embodiments, the component 50 substantially fillsthe entire recess 48, as shown in FIG. 14. However, in some embodimentsa component 52 is sized such that it fills only a portion of the recess48, as shown in FIG. 15. In such embodiments, the remaining portion ofthe recess 44 may remain vacant—as shown in FIG. 15—or the remainingopen portion may be filled with another material or component. In someembodiments, the component 52 is secured in the recess 48 by theintroduction of additional material into the open space remaining in therecess.

Referring now to FIGS. 16 and 17, shown therein is a prosthetic device60 according to another embodiment of the present disclosure. In someinstances, the prosthetic device 60 is substantially similar to theprosthetic device 10 described above. Accordingly, similar referencenumerals may be utilized and the description limited for particularaspects of the prosthetic device 60. However, it should be noted thatthe prosthetic device 60 does not include a fixation member 16 or afixation device 18 as described with respect to the prosthetic device10. Rather, the prosthetic device 60 is configured to be implantedwithout rigid fixation to either the femur or tibia. In this regard, theprosthetic device 60 may be implanted into a patient without causingpermanent damage to the patient's tibia or other bone structure(s)engaged by the prosthetic device. Accordingly, the prosthetic device 60may be implanted in an attempt to alleviate the patient's knee problemswhile avoiding permanent destruction of the patient's anatomy, such ascutting or reaming a large opening in the tibia. more invasiveprocedures.

To this end, the prosthetic device 60 includes a boundary 62 extendingbetween the ends of the outer body portion 12 having an increased heightrelative to the central body portion 14. In that regard, the boundary 62extends substantially above an upper surface 63 of the central bodyportion 14. Accordingly, the outer body portion 12 and the boundary 62completely surround the central body portion 14. Accordingly, whenpositioned between the femur and tibia the outer body portion 12 and theboundary 62 define the outer limits of movement for the femur relativeto the prosthetic device 60. That is, the increased height of the outerbody portion 12 and the boundary 62 along with the contact pressure onthe prosthetic device 60 from being positioned between the femur and thetibia prevents the femur from escaping the cavity defined by the outerbody portion 12 and the boundary 62. In some embodiments, the boundary62 has a thickness that is between ¼ and ¾ the total thickness or heightof the outer body portion 12. In one particular embodiment, the boundary62 has thickness such that it extends above the upper surface 63approximately one-half of the distance of the upper surface 24 of theouter body portion 12.

Further, the upper surface 63 and the lower surface 64 are botharticulating bearing surfaces in the current embodiment. In particular,the upper and lower surfaces 63, 64 are configured to movingly engagewith the femur and tibia, respectively. In that regard, the prostheticdevice 60 can translate and rotate with respect to the femur and/ortibia. Translation is possible in both the anterior-posterior andmedial-lateral directions. In some embodiments, the upper surface 63includes both a vertical and a horizontal bearing component. To thatend, in some embodiments the upper surface 63 comprises a concavesurface that defines the vertical and horizontal bearing components.Similarly, in some embodiments the lower surface 64 includes both avertical and horizontal bearing component. In particular, in someembodiments the lower surface 64 comprises a convex surface. In otherembodiments, the lower surface 64 comprises only a vertical bearingcomponent and is substantially planar. In such embodiments, the tibiamay be prepared to mate with the substantially planar lower surface. Insome embodiments, the upper surface 63 and/or the lower surface 64 areshaped such that the prosthetic device 10 is biased towards a neutralposition in the knee. For example, the arcuate profiles of the uppersurface 63 and/or the lower surface 64 are shaped such that theinteraction between the surfaces and the bone encourages the bone to aparticular orientation relative to the surfaces.

Referring to FIG. 18, in some embodiments the prosthetic device 10includes one or more recesses 66 in the upper surface 20 that providefor the accumulation of synovial fluid. In some embodiments, therecesses 66 are positioned at the most prevalent contact points of thefemur with the upper surface 20. In such embodiments, the synovial fluidlubricates the upper articulation surface 20 of the prosthetic device.The recesses 66 may have various shapes within the upper surface 20. Inthat regard, the recesses 66 may comprises a sloping depression thatcreates a concave recess in some embodiments. The concave recess maycomprises a substantially circular profile, an elongated profile, anirregular shape, and/or combinations thereof. In the current embodiment,the recesses 66 are shown as comprising a D-shaped profiles spacedequally about a midline of the prosthetic device 10. However, thisdepiction is not to be limiting as many shapes, sizes, and transitionsmay be utilized for the recesses 66. In other embodiments, theprosthetic device 10 includes a greater or fewer number of recesses 66.In some embodiments, the prosthetic device 10 does not include anyrecesses in the upper surface 20.

In some instances, the femoral condyle may be surgically prepared topermit near-normal knee joint flexion after implantation. Similarly, thetibial plateau may be surgically prepared to fixedly engage with theprosthetic devices.

A variety of materials are suitable for use in making the prostheticmeniscus. Medical grade polyurethane based materials especially suitablefor use in the embodiments described include, but are not limited to thefollowing:

Bionate®, manufactured by POLYMER TECHNOLOGY GROUP PTG, apolycarbonate-urethane is among the most extensively tested biomaterialsever developed. Carbonate linkages adjacent to hydrocarbon groups givethis family of materials oxidative stability, making these polymersattractive in applications where oxidation is a potential mode ofdegradation, such as in pacemaker leads, ventricular assist devices,catheters, stents, and many other biomedical devices. Polycarbonateurethanes were the first biomedical polyurethanes promoted for theirbiostability. Bionate® polycarbonate-urethane is a thermoplasticelastomer formed as the reaction product of a hydroxyl terminatedpolycarbonate, an aromatic diisocyanate, and a low molecular weightglycol used as a chain extender. The results of extensive testingencompassing Histology, Carcinogenicity, Biostability, and TripartiteBiocompatibility Guidance for Medical Devices verifies the costeffective material's biocompatibility.

Another group of potentially suitable materials are copilymers ofsilicone with poplyurethanes as exemplified by PurSil™, a SiliconePolyether Urethane and CarboSil™, a Silicone Polycarbonate Urethane.Silicones have long been known to be biostable and biocompatible in mostimplants, and also frequently have the low hardness and loq modulususeful for many device applications. Conventional silicone elastomerscan have very high ultimate elongations, but only low to moderatetensile strengths. Consequently, the toughness of most biomedicalsilicone elastomers is not particularly high. Another disadvantage ofconventional silicone elastomers in device manufacturing is the need forcross-linking to develop useful properties. Once cross-linked, theresulting thermoset silicone cannot be redissolved or remelted. Incontrast, conventional polyurethance elastomers are generallythermoplastic with excellent physical properties. Thermoplastic urethaneelastomers (TPUs) combine high elongation and high tensile strength toform tough, albeit fairly high-modulud elastomers. Aromatic polyetherTPUs can hica excellent flex life, tensile strength exceeding 5000 psi,and ultimate elongations greater than 700 percent. The are often usedfor continuously flexing, chronic implants such as ventricular-assistdevices, intraaortic balloons, and artificial heart components. TPUs caneasily be processed by melting or dissolving the polymer to fabricate itinto useful shapes.

The prospect of combining the biocompatibility and biostability ofconventional silicone elastomers with the processability and toughnessof TPUs is an attractive approach to what would appear to be a nearlyideal biomaterial. For instance, it has been reported that silicone actssynergistically with both polycarbonate- and polyether-basedpolyurethanes to improve in vivo and in vitro stability. Inpolycarbonate-based polyurethanes, silicone copolymerization has beenshown to reduce hydrolytic degradation of the carbonate linkage, whereasin polyether urethanes, the covalently bonded silicone seems to protectthe polyether soft segment from oxidative degradation in vivo.

POLYMER TECHNOLOGY GROUP PTG synthesized and patentedsilicone-polyurethane copolymers by combining two previously reportedmethods: copolymerization of silicone (PSX) together with organic(non-silicone) soft segments into the polymer backbone, and the use ofsurface-modifying end groups to terminate the copolymer chains.Proprietary synthesis methods make high-volume manufacturing possible.

Other potentially applicable materials include PurSil™silicone-polyether-urethane and CarboSil™silicone-polycarbonate-urethane which are true thermoplastic copolymerscontaining silicone in the soft segment. These high-strengththermoplastic elastomers are prepared through a multi-step bulksynthesis where polydimethylsiloxane (PSX) is incorporated into thepolymer soft segment with polytetramethyleneoxide (PTMO) (PurSil) or analiphatic, hydroxyl-terminiated polycarbonate (CarboSil). The hardsegment consists of an aromatic diisocyanate, MDI, with low molecularweight glycol chain extender. The copolymer chains are then terminatedwith silicone (or other) Surface-Modifying End Groups™. Aliphatic (AL)versions of these materials, with a hard segment synthesized from analiphatic diisocyanate are also available.

Many of these silicone urethanes demonstrate previously unavailablecombinations of physical properties. For example, aromatic siliconepolyetherurethanes have a higher modulus at a given shore hardness thanconventional polyether urethanes—the higher the silicone content, thehigher the modulus (see PurSil Properties). Conversely, the aliphaticsilicone polyetherurethanes have a very low modulus and a high ultimateelongation typical of silicone homopolymers or even natural rubber (seePurSil AL Prperties). This makes them very attractive ashigh-performance substitutes for conventional cross-linked siliconerubber. In both the PTMO and PC families, certain polymers have tensilestrengths three to five times higher than conventional siliconebiomaterials.

Further examples of suitable materials include Surface Modifying EndGroups™ (SMEs) which are surface-active oligomers covalently conded tothe base polymer during synthesis. SMEs—which include silicone (S),sulfonate (SO), fluorocarbon (F), polyethylene oxide (P), andhydrocarbon (H) groups—control surface chemistry without compromisingthe bulk properties of the polymer. The result is key surfaceproperties, such as thromboresistance, biostability, and abrasionresistance, are permanently enhanced without additional post-fabricationtreatments or topical coatings. This patented technology is applicableto a wide range of PTG's polymers.

SMEs provide a series of (biomedical) base polymers that can achieve adesired surface chemistry without the use of additives. Polyurethanesprepared according to PTG's development process couple endgroups to thebackbone polymer during synthesis via a terminal isocyanate group, not ahard segment. The added mobility of endgroups relative to the backboneis though to facilitate the formation of uniform overlayers by thesurface-active (end) blocks. The use of the surface active endgroupsleaves the original polymer backbone intact so the polymer retainsstrength and processability. The fact that essentially all polymerschains carry the surface-modifying moiety eliminates many of thepotential problems associated with additives.

The SME approach also allows the incorporation of mixed endgroups into asingle polymer. For example, the combination of hydrophobic andhydrophilic endgroups gives the polymers amphipathic characteristics inwhich the hydrophobic versus hydrophilic balance may be easilycontrolled.

Other suitable materials, manufactured by CARDIOTECH CTE, includeChronoFlex™ And Hydrothane™.

The ChronoFlex®, polycarbonate aromatic polyurethanes, family ofmedical-grade segmented biodurable polyurethane elastomers have beenspecifically developed by CardioTech International to overcome the invivo formation of stress-induced microfissures. HydroThane™, hydrophilicthermoplastic polyurethanes, is a family of super-absorbent,thermoplastic, polyurethane hydrogels ranging in water content from 5 to25% by weight. HydroThane™ is offered as a clear resin in durometerhardness of 80 A and 93 Shore A. The outstanding characteristic of thisfamily of materials is the ability to rapidly absorb water, high tensilestrength, and high elongation. The result is a polymer having somelubricious characteristics, as well as being inherently bacterialresistant due to their exceptionally high water content at the surface.HydroThane™ hydrophilic polyurethane resins are thermoplastic hydrogels,and can be extruded or molded by conventional means. Traditionalhydrogels on the other hand are thermosets and difficult to process.

Additional suitable materials manufactured by THERMEDICS includeTecothante® (aromatic polyether-based polyurethane), Carbothane®(aliphatic polycarbonate-based polyurethane), Tecophilic® (high moistureabsorption aliphatic polyether-based polyurethane) and Tecoplast®(aromatic polyether-based polyurethane). Tecothane® is a family ofaromatic, polyether-based TPU's available over a wide range ofdurometers, colors, and radiopacifiers. One can expect Tecothane resinsto exhibit improved solvent resistance and biostability when comparedwith Tecoflex resins of equal durometers. Carbothane® is a family ofaliphatic, polycarbonate-based TPU's available over a wide range ofdurometers, colors and radiopacifiers. This type of TPU has beenreported to exhibit excellent oxidative stability, a property which mayequate to excellent long-term biostability. This family, like Tecoflex,is easy to process and does not yellow upon aging. Tecophilic® is afamily of aliphatic, polyether-based TPU's which have been speciallyformulated to absorb equilibrium water contents of up to 150% of theweight of dry resin.

Polyurethanes are designated aromatic or aliphatic on the basis of thechemical nature of the diisocyanate component in the formulation.Tecoflex, Tecophilic and Carbothane resins are manufactured using thealiphatic compound, hydrogenated methylene diisocyanate (HMDI).Tecothane and Tecoplast resins use the aromatic compound methylenediisocyanate (MDI). Tecoflex® is a family of aliphatic, polyether-basedTPU's. These resins are easy to process and do not yellow upon aging.Solution grade versions are candidates to replace latex. Allformulations, with the exception of Carbothane, are formulated usingplytetramethylene ether glycol (PTMEG) and 1,4 butanediol chainextender. Carbothane is specifically formulated with a polycarbonatediol (PCDO). These materials represent the major chemical compositiondifferences among the various families. Aromatic and aliphaticpolyurethanes share similar properties that make them outstandingmaterials for use in medical devices. In general, there is not muchdifference between medical grade aliphatic and aromatic polyurethaneswith regard to the following chemical, mechanical and biologicalproperties: high tensile strength (4,000 to 10,000 psi); high ultimateelongation (250 to 700%); wide range durometer (72 Shore A to 84 ShoreD); good biocompatibility; high abrasion resistance; good hydrolyticstability; can be sterilized with ethylene oxide and gamma irradiation;retention of elastomeric properties at low temperature; good meltprocessing characteristics for extrusion, injection molding, etc.

With such an impressive array of desirable features, it is no wonderthat both aliphatic and aromatic polyurethanes have become increasinglythe material of choice in the design of medical grade components. Thereare, however, distinct differences between these tow families ofpolyurethane that could dictate the selection of one over the other fora particular application:

In their natural states, both aromatic and aliphatic polyurethanes areclear to very light yellow in color. Aromatics, however, can turn darkyellow to amber as a result of melt processing or sterilization, or evenwith age. Although the primary objection to the discoloration ofaromatic clear tubing or injection molded parts is aesthetic; theyellowing, which is caused by the formation of a chromophore in the NMIportion of the polymer, does not appear to affect other physicalproperties of the material. Radiopaque grades of Tecothane also exhibitsome discoloration during melt processing or sterilization. However,both standard and custom compounded radiopaque grades of Tecothane havebeen specifically formulated to minimize this discoloration.

Aromatic polyurethanes exhibit better resistance to organic solvents andoils than do aliphatics—especially as compared with low durometer (80 to85 Shore A) aliphatic, where prolonged contact can lead to swelling ofthe polymer and short-term contact can lead to surface tackiness. Whilethese effects become less noticeable at higher durometers, aromaticsexhibit little or no sensitivity upon exposure to the common organicsolvents used in the health care industry.

Both aliphatic and aromatic poly-ether based polyuretanes softenconsiderably within minutes of insertion in the body. Many devicemanufacturers promote this feature of the urethane products because ofpatient comfort advantage as well as the reduced risk of vasculartrauma. However, this softening effect is less pronounced with aromaticresins than with aliphatic resins.

Tecothane, Tecoplast and Carbothane melt at temperatures considerablyhigher than Tecoflex and Tecophilic. Therefore, processing by eitherextrusion of injection molding puts more heat history into productsmanufactured from Tecothane, Tecoplast and Carbothane. For example,Tecoflex EG-80A and EG-60D resins mold at nozzle temperatures ofapproximately 310 degrees F. and 340 degrees F. respectively whileTecothane and Carbothane products of equivalent durometers mold atnozzle temperatures in the range of 380 degrees F. and 435 degrees F.

Additional materials of interest include Tecogel, a new member to theTecophilic family, a hydrogel that can be formulated to absorbequilibrium water contents between 500% to 2000% of the weight of dryresin, and Tecoplast®, a family of aromatic, polyether-based TPU'sformulated to produce rugged injection molded components exhibiting highdurometers and heat deflection temperatures.

Additional potentially suitable materials include four families ofpolyurethanes, named Elast-Eon™, which are available from AorTechBiomaterials.

Elast-Eon™ 1, a Polyhexamethylene oxide (PFMO), aromatic polyurethane,is an improvement on conventional polyurethane in that it has a reducednumber of the susceptible chemical groups. Elast-Eon™ 2, a Siloxanebased macrodiol, aromatic polyurethane, incorporates siloxane unto thesoft segment. Elast-Eon™ 3, a Siloxane based macrodiol, modified hardsegment, aromatic polyurethane, is a variation of Elast-Eon™ 2 withfurther enhanced flexibility due to incorporation of siloxane into thehard segment. Elast-Eon™ 4 is a modified aromatic hard segmentpolyurethane.

Bayer Corporation also produces candidate materials. Texin 4210 andTexin 4215 are thermoplastic polyurethane/polycarbonate blends forinjection molding and extrusion. Texin 5250, 5286 and 5290 are aromaticpolyether-based medical grade materials with Shore D hardness ofapproximately 50, 86, and 90 respectively for injection molding andextrusion. They each comply with 21 CFR 177.1680 and 177.2600.

It should be appreciated that the devices described hereinabove, whilepreferably formed by injection molding of polyurethane, may also beformed by any suitable manufacturing method and may be formed of anysuitable medical grade elastomers. It is further appreciated that any ofthe following manufacturing methods may be utilized: injection moldingincluding inserting inserts, compression molding including insertinginserts, injection-compression molding including inserting inserts,compression molding of prefabricated elements pre-formed by any of theabove methods including inserting inserts, spraying including insertinginserts, dipping including inserting inserts, machining from stocks orrods, machining from prefabricated elements including inserting inserts.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

While the present disclosure has been particularly shown and describedwith reference to particular embodiments, it will be fully appreciatedthat variations of the above-disclosed and other features and functions,or alternatives thereof, may be desirably combined into many otherdifferent systems or applications. Also that various presentlyunforeseen or unanticipated alternatives, modifications, variations ofimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

1-17. (canceled)
 18. A prosthetic device for replacing a naturalmeniscus, the prosthetic device comprising: a central body portioncomprising a flexible polymeric material, the central body portionhaving an upper articulating bearing surface for movingly engaging afemoral condyle and a lower articulating bearing surface for movinglyengaging a tibial plateau; an outer body portion completely surroundingthe central body portion and having an increased thickness with respectto the central body portion, the outer body portion configured to limitmovement of the femoral condyle with respect to the upper articulatingbearing surface of the central body portion, the outer body portionhaving an increased stiffness with respect to the central body portion;wherein the central body portion and the outer body portion comprise amonolithic structure; and wherein the prosthetic device is configuredfor implantation without removing any portion of the tibial plateau. 19.The prosthetic device of claim 18, wherein a majority of the outer bodyportion has a first thickness and a minority of the outer body portionhas a second thickness, the first thickness being greater than thesecond thickness.
 20. The prosthetic device of claim 19, wherein thesecond thickness is approximately half of the first thickness.
 21. Theprosthetic device of claim 18, wherein the upper surface comprises aconcave surface and the lower surface comprises a convex surface. 22.The prosthetic device of claim 18, wherein the outer body portioncomprises a filament extending therethrough, the filament increasingoverall the stiffness of the outer body portion.
 23. (canceled)